<p>Anything can happen, so there is no harm to holding your place on a waiting list. Those are three highly-selective schools, and so you can’t expect many places to open up on the waiting list, but they compete for many of the same exceptional students. I don’t know whether any of them “over-accept,” the way all but the most selective colleges do; I suspect they do not, and so some students are inevitably invited from the waiting list. You would probably have decided on a school had any of them accepted your son, and it’s safe to presume that most accepted students will. In that case, the waiting lists shrink as students who have been accepted elsewhere commit to their schools.
Don’t lose hope, but don’t expect to beat the odds either. If a call comes in the next five months, allow it to be an epiphany, but proceed with life otherwise.</p>